Whereas the exploration of urban expansion only from the horizontal perspective cannot accurately reflect the real situation, vertical expansion studies can fully understand the characteristics and rules for urban expansion. Understanding how the urban built-up density based on the vertical expansion differs across cities and varies among regions over time is important for enriching the research on urban expansion, as such work can provide theoretical support for the sustainable use of urban land and urban sustainability. This research uses the floor area ratio (FAR) of newly residential land in 320 prefecture-level cities in China from 2007 to 2016 as an urban residential built-up density index to explore the spatiotemporal characteristics, regional differences and influencing factors of vertical urban expansion. The conclusions are as follows: the FAR of urban residential land was 2.42 in 2016, and the urban residential built-up density was increasing. The spatial pattern of the urban residential built-up density in China is an inverted U-shape in the longitudinal direction and decreases from north to south in the latitudinal direction. This density roughly follows the pattern of Central China > West China > East China > Northeast China with remarkable regional differences, and the trend of spatial agglomeration is strengthening. At the national level, the urban residential built-up density is significantly positively correlated with the local government’s motives, the residential developer’s intensions and the consumption capacity of residents, and there is a significant inverse U-shaped relationship between the average wage and the FAR. Finally, the findings indicate that we should grasp the urban expansion characteristics from both horizontal and vertical expansion. The government should take the construction of a livable and resilient city as the goal in land supply and urban planning. In addition, the government should formulate reasonable land supply policies and urban development layouts to promote sustainable urban land use as well as the healthy development of cities in the future.